This invention relates to protection apparatus for electrical equipment and particularly to protection of series capacitors.
Reference is made to copending applications Ser. No. 43,531, filed May 29, 1979 by C. A. Peterson, et al., now U.S. Pat. 4,295,174, Oct. 13, 1981; and Ser. No. 126,094, filed Feb. 29, 1980 by C. A. Person, now abandoned, both of which are assigned to the present assignee, for background description in relation to the application of non-linear resistors, particularly those of the metal oxide varistor type, to the protection of electrical power apparatus, particularly series capacitors. Size, cost, and availability constraints make it necessary to provide a plurality, frequently ten or more, varistor assemblies electrically in parallel across the equipment to be protected in order to obtain a sufficient level of current carrying capacity and energy absorption capability upon occurrence of a fault condition. It is intended that the various paralleled resistor assemblies have matching characteristics. Ideally, they would all turn on and become highly conductive upon the same overvoltage being impressed upon them. This would result in their common operation so that the current resulting from the applied voltage would be equally shared. If so, the design of the equipment would be relatively straightforward to provide a sufficient number of parallel units to handle the current produced and the energy required to be absorbed. In practice, this ideal situation is not obtainable because of inherent small variations in characteristics of individual varistors. This requires a design involving an excess number of units to be applied to the protection equipment so as to provide the necessary insurance for proper functioning. Any such excess units of course entails additional cost which is desirably to be avoided.
The problem being addressed here is that of "energy balancing" among the several parallel varistor elements in the protective apparatus. The problem results from the slight manufacturing variability of characteristics of the varistors, such as those essentially of zinc oxide and other metal oxides, which result in a large variation of current among parallel varistors under maximum fault current conditions. The non-linearity characteristic of such device is expressed by the equation EQU .alpha.=[1n(I.sub.2 /I.sub.1)]/[1n(E.sub.2 /E.sub.1)]
in which the coefficient .alpha. represents the degree of non-linearity. It is particularly difficult to fabricate a large number of resistor assemblies in which the .alpha. values are exactly the same. The higher the .alpha. value, the higher the potential current imbalance among various resistors having the same variation in .alpha.. That is, for device of interest having an .alpha. of at least about 30, a variation in discharge voltage of .+-.1%, which is about the practical limit of device fabrication in accordance with the present state of the art, may produce a current variation between parallel units of about 15-20%. This imbalance requires an extra margin of protective elements to be applied to the equipment to achieve the required overall energy absorption capability. Roughly speaking in terms of energy required to be absorbed, there may be required up to about 20% additional energy absorption capability, as compared to an ideal case of precisely matched characteristics, which results in a requirement of 20% additional pounds of varistor material and related cost.
The purposes of this invention have to do with being able to tolerate the inherent variation of characteristics between relatively well made metal oxide varistor units and at the same time being able to utilize fewer such units in achieving the required functional performance for the protection of high voltage equipment such as series capacitors.
Briefly, the invention achieves the foregoing objectives by connecting a resistor having a positive temperature coefficient (PTC) of resistance in series with each of the varistor assemblies. The resistor elements, which may comprise merely a wire of material such as Nichrome alloy will under a fault condition heat up and increase in resistance as a function of I.sup.2 t through each of the respective varistors. Therefore, a varistor whose inherent characteristics are such that it would tend on its own to carry more current than another parallel varistor, would heat the wire up more and cause an increase in the resistance of the affected series connected wire. This would reduce the current flow to the affected varistor and shift the current to adjacent varistors that have lower impedance. The current and the consequent joules of energy would therefore be forced dynamically to be shared among all of the parallel assemblies. This ensures against overburdening an individual assembly which could result in damage to it.
The invention is attractive for application to protecting series capacitors because they normally require a relatively large number of varistors. It is desirable that the selection of the individual voltage limiters be not critical as to the slope of their non-linear voltage-current characteristic and hence the invention will reduce the matching and testing of zinc oxide limiter units as well as minimize the numbers thereof required in the total assembly. Also, the positive temperature coefficient resistance wire employed can be readily incorporated in the equipment at relatively low cost. The PTC will introduce almost no losses under normal operating conditions because the current through the varistor-resistor combination will be less than a milliampere.
Although this application of the PTC resistors will result in a slight reduction of the .alpha. value of the series combination of the wire and the varistor (e.g. 2-4%), a result which is unfavorable in itself, a point can be selected at which the tradeoff is very favorable because the higher the resistance of the connected series wire, the greater is the energy balancing effect. For example, the energy balance may be improved from .+-.30% to .+-.10%.